Stress corrosion cracking (SCC) is a known phenomenon occurring in reactor components, such as structural members, piping, fasteners, and welds, exposed to high-temperature water. As used herein, SCC refers to cracking propagated by static or dynamic tensile stressing in combination with the reactor aqueous environment. The reactor components are subject to a variety of stresses associated with, e.g., differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stress from welding, cold working and other asymmetric metal treatments. In addition, water chemistry, welding, crevice geometry, heat treatment, and radiation can increase the susceptibility of metal in a component to SCC.
It is well known that SCC occurs at higher rates when oxygen is present in the reactor water in concentrations of about 5 ppb or greater. SCC is further increased in a high radiation flux where oxidizing species, such as oxygen, hydrogen peroxide, and short-lived radicals, are produced from radiolytic decomposition of the reactor water. Such oxidizing species increase the electrochemical corrosion potential (ECP) of metals. Electrochemical corrosion is caused by a flow of electrons from anodic to cathodic areas on metallic surfaces. The ECP is a measure of the kinetic tendency for corrosion phenomena to occur, and is a fundamental parameter in determining rates of, e.g., SCC, corrosion fatigue, corrosion film thickening, and general corrosion.
In a BWR, the radiolysis of the primary water coolant in the reactor core causes the net decomposition of a small fraction of the water to the chemical products H.sub.2, H.sub.2 O.sub.2, O.sub.2 and oxidizing and reducing radicals. For steady-state operating conditions, equilibrium concentrations of O.sub.2, H.sub.2 O.sub.2, and H.sub.2 are established in both the water which is recirculated and the steam going to the turbine. This concentration of O.sub.2, H.sub.2 O.sub.2, and H.sub.2 is oxidizing and results in conditions that can promote intergranular stress corrosion cracking (IGSCC) of susceptible materials of construction. One method employed to mitigate IGSCC of susceptible material is the application of hydrogen water chemistry (HWC), whereby the oxidizing nature of the BWR environment is modified to a more reducing condition. This effect is achieved by adding dissolved hydrogen to the reactor feedwater. When the hydrogen reaches the reactor vessel, it reacts with the radiolytically formed oxidizing species on metal surfaces to reform water, thereby lowering the concentration of dissolved oxidizing species in the water in the vicinity of metal surfaces. The rate of these recombination reactions is dependent on local radiation fields, water flow rates and other variables.
The injected hydrogen reduces the level of oxidizing species in the water, such as dissolved oxygen, and as a result lowers the ECP of metals in the water. However, factors such as variations in water flow rates and the time or intensity of exposure to neutron or gamma radiation result in the production of oxidizing species at different levels in different reactors. Thus, varying amounts of hydrogen have been required to reduce the level of oxidizing species sufficiently to maintain the ECP below a critical potential required for protection from IGSCC in high-temperature water. As used herein, the term "critical potential" means a corrosion potential at or below a range of values of about -0.230 to -0.300 V based on the standard hydrogen electrode (SHE) scale. IGSCC proceeds at an accelerated rate in systems in which the ECP is above the critical potential, and at a substantially lower or zero rate in systems in which the ECP is below the critical potential. Water containing oxidizing species such as oxygen increases the ECP of metals exposed to the water above the critical potential, whereas water with little or no oxidizing species present results in an ECP below the critical potential.
It has been shown that IGSCC of Type 304 stainless steel (containing 18-20% Cr, 8-10.5% Ni, 2% Mn, remainder Fe) used in BWRs can be mitigated by reducing the ECP of the stainless steel to values below -230 mV(SHE). An effective method of achieving this objective is to use HWC. However, high hydrogen additions, e.g., of about 200 ppb or greater, that may be required to reduce the ECP below the critical potential, can result in a higher radiation level in the steam-driven turbine section from incorporation of the short-lived N-16 species in the steam. For most BWRs, the amount of hydrogen addition required to provide mitigation of IGSCC of pressure vessel internal components results in an increase in the main steam line radiation monitor by a factor of five to eight. This increase in main steam line radiation can cause high, even unacceptable, environmental dose rates that can require expensive investments in shielding and radiation exposure control. Thus, recent investigations have focused on using minimum levels of hydrogen to achieve the benefits of HWC with minimum increase in the main steam radiation dose rates.
An effective approach to achieve this goal is to coat, alloy or dope the stainless steel surface with palladium or other noble metal. The techniques used to date for palladium coating include electroplating, electroless plating, hyper-velocity oxy-fuel, plasma deposition and related high-vacuum techniques. Palladium alloying has been carried out using standard alloy preparation techniques.
The most critical requirement for IGSCC protection of Type 304 stainless steel is to lower its ECP to values below the protection potential, i.e., -0.230 V(SHE). The manner in which this potential is achieved is immaterial, e.g., by alloying, doping or by any other method. It has been demonstrated that it is sufficient to dope the oxide film by the appropriate material (e.g., Pd) to achieve a state of lower ECP. It was shown in later work that a thickness of 200 to 300 .ANG. of the doping element (Pd) is sufficient to impart this benefit of lower potential.
The presence of palladium on the stainless steel surface reduces the hydrogen demand to reach the required IGSCC protection potential of -0.230 V(SHE). It has been shown that Pd-containing stainless steel surfaces reach a negative potential of -0.500 V(SHE) as soon as the H.sub.2 /O.sub.2 molar ratio exceeds the stoichiometric value of 2 (see FIG. 1). The exact potential of the Pd-containing stainless steel surface is determined by the Nernst equation depending on the system temperature and the dissolved excess hydrogen content in the water. The presence of palladium on the surface provides sufficient catalytic activity for H.sub.2 and O.sub.2 recombination to occur, which reduces the ECP of stainless steel surfaces below the potential required for IGSCC mitigation.
If noble metal technology involving coating, alloying or doping of noble metals such as palladium is used for IGSCC protection of stainless steels, then it is important to know the hydrogen injection levels required to reach stoichiometric or super-stoichiometric (i.e., H.sub.2 /O.sub.2 molar ratios of 2 or greater) conditions.
The primary method used to quantify the levels of hydrogen injection needed to achieve IGSCC protection is the measurement of the ECP of the surface of interest in BWR water in a specific region. Electrochemical corrosion potential (ECP) monitoring is conventionally carried out employing paired electrochemical half-cell probes or electrodes which are mounted within the recirculation piping or in an external vessel which has its water source from the reactor water in the recirculation piping. The electrodes are accessed to the external environment through gland-type mountings or the like. Where the electrode system of interest involves the potential from a metal corrosion electrode, then the reference electrode can conveniently be a metal in equilibrium with a slightly soluble salt of the metal, which in turn is in equilibrium with a soluble salt having the same anion as the insoluble salt. The metal salt couple must be chemically stable. Accordingly, one of the thus-mounted probes which is configured as a reference electrode may be based, for example, on a silver/silver chloride half-cell reaction. Once the reference electrode half-cell is defined, the cell is completed with the sensing cell portion based upon a metal whose potential is to be measured, such as platinum or stainless steel. Verification of the reference electrode and/or the electrode pair is carried out by thermodynamic evaluation and appropriate Nernst equation-based electrochemical calculations in combination with laboratory testing within a known environment. When the measured ECP and value calculated from thermodynamics are in agreement, verification of the reference electrode performance is achieved.
Half-cell electrodes developed for use in reactor circulation piping traditionally have been configured with metal housings, high-temperature ceramics and polymeric seals such as Teflon brand polytetrafluoroethylene. These structures have performed adequately in the more benign and essentially radiation-free environments of recirculation piping.
over the recent past, investigators have sought to expand the ECP monitoring procedures to the severe environment of the fluid in the vicinity of the reactor core itself for the purpose of studying or quantifying the effect of hydrogen water chemistry adjustment in mitigating irradiation-assisted stress corrosion cracking (IASSC) as well as IGSCC. Within the reactor core, the monitoring electrode can be mounted, for example, with otherwise unemployed or in tandem with the traveling instrumentation probe of available local power range monitors and the like. The monitors are located in a severe, high-temperature (550.degree. F.), high-radiation (typically 10.sup.9 R (rads) per hour gamma, 10.sup.13 R per hour neutron) water environments.
Thus, conventional ECP monitoring of BWR components has required the installation of at least two separate half-cell electrodes, namely, a platinum sensor that responds to hydrogen in the BWR water and a silver/silver chloride reference electrode that does not respond to hydrogen, with their associated cables. It would be an advance in the art if ECP monitoring could be performed using a sensor configuration requiring installation of only one cable for sensing and signaling the point of reversibility reaching stoichiometry with respect to hydrogen.